The present invention relates to the field of cross-connect switches and more particularly to a cross-connect switch for automating the functions of a main distribution frame in a telephone system.
In a typical telephone system, each subscriber's telephone is connected to a central office by a subscriber loop consisting of two metallic conductors which, for historical reasons, are referred to as "tip" and "ring". The subscriber loops are organized into cables which are routed past the premises of potential subscribers. In each central office, the subscriber loops serviced by that office terminate on the main distribution frame.
The main distribution frame is used to make assignments between the subscriber loops and the central office switch input lines. The subscriber loops are connected to terminals on one side of the main distribution frame, and the input lines to the central office switch are connected to terminals on the other side of the main distribution frame. Assignments are made by manually connecting the tip and ring conductors of a subscriber loop to the tip and ring conductors of the desired input line using two metallic conductors. Hence, the main distribution frame represents a cross-connect field which grows in complexity as new subscriber loops are connected to the central office switch. As the complexity of this cross-connect field increases, the maintenance costs associated with it also increase; hence, automation of this function is desirable.
In spite of numerous improvements in telephone switching, the main distribution frame has remained essentially as described above for over 50 years. The reasons for this lie in certain advantages bestowed by this simple structure. First, any subscriber loop can be connected to any central office switch input line. Second, the main distribution frame provides a convenient point for making physical connections to any of the subscriber loops for the purpose of testing the loops. Finally, the capital investment in the main distribution frame is relatively small compared with the alternative systems devised over the years.
There have been a number of systems proposed for automating the functions of the main distribution frame. Joel, Jr. (U.S. Pat. No. 3,562,435) teaches a cross-connect switch for this function. However, the cost of this system has been too great for it to be practical.
Kapel, et al. (U.S. Pat. No. 3,763,325) also teach a cross-connect switch for automating the main distribution frame functions which reduces the cost of cross-connect switch by using a cross-connect switch consisting of a number of terminal strips which are manually wired by crafts people in response to instructions generated by data processing computers which are not a part of the invention in question. The manual labor required to make the connections in this switch make the switch unattractive for automating the main distribution frame functions.
Bergeron, Jr., et al. (U.S. Pat. No. 3,978,291) describe a cross-connect switch for automating the main distribution frame functions which employs a switch module in which connections are made by physically placing pins into circuit boards. The operation of pin placement is carried out by robotics. This system may be viewed as that of Kapel, et al. with the manual circuit wiring having been replaced by an automated system. This system has also not found wide spread use because of the costs associated with it.
Each of the above mentioned cross-connect switches has the same number of output lines as input lines. Such designs are poorly suited for automating a main distribution frame. In general, only a fraction of the subscriber loops terminating in the central office are in use at any given time. The remaining subscriber loops represent subscriber loops which were connected to a subscriber who has terminated service or subscriber loops which are spares that have not yet been connected to any subscriber's premises. As a result, there are more subscriber loops in place than central office switch inputs to service subscriber loops. Hence, a cross-connect switch for automating the main distribution frame function should have significantly more output lines for connection to subscriber loops than input lines for connection to the central office switch. The above mentioned prior art cross-connect switches consisted of cross-connect switches having one input line for each subscriber loop. In these cross-connect switches the number of lines must be chosen to be the maximum number of subscriber loops which are connected to the central office. As a result, the cross-connect switches have many more inputs for connection to the central office switch than there are central office switch lines. These excess inputs and the switch or cross points needed to implement them substantially increase the cost of the cross-connect switches.
In the case of the system taught by Bergeron, et al. the number of input and output lines are further restricted. Bergeron, et al. teach a cross-connect switch in which the number of input lines must be equal to K.sup.3 where K is an integer. If the desired number of subscriber loops is not equal to the cube of an integer, one must build an even bigger cross-connect switch. That is, one must build a switch with K.sup.3 inputs where K is chosen such that K.sup.3 is greater than the desired number of lines and (K-1).sup.3 is less than the desired number of lines. This restriction, in general, further increases the cost of the cross-connect switch, since it requires that further unneeded capacity be included in the switch whenever the optimum number of lines is not equal to the cube of an integer.
A second problem with these prior art designs relates to the size of the basic switch module used to construct the cross-connect switches in question. In general, a cross-connect switch consists of a number of stages. Each stage is constructed from a plurality of switch modules. Each switch module has a plurality of input and output lines. Each switch module is constructed from a number of switch points, commonly referred to as cross-points, which allow the module to be used to connect any of its input lines to any of its output lines, provided the input and output lines in question are not already connected.
In general, the repeated element from which cross-connect switches are constructed is a switch module, not a switch point. Hence, if one is to minimize the cost of the cross-connect switch, the cross-connect switch design must obtain the maximum economics of scale in constructing the switch modules as well as minimizing the total number of switch points in the cross-connect switch. The prior art designs have employed relatively large switch modules. For example, the cross-connect switch taught by Bergeron, Jr., et al. employs a large switch module having 64 input and 64 output lines. If large switch modules are used, the number of such switch modules in a cross-connect switch will be too small to obtain the full economies of scale. In addition, the number of switch points needed to implement the cross-connect switch is much greater than would be needed if a smaller switch module had been chosen.
Broadly, it is an object of the present invention to provide an improved cross-connect switch which is adapted for automating the functions currently provided by the main distribution frame in a telephone system.
It is a further object of the present invention to provide a cross-connect switch which may be constructed from one or two repeated switch modules.
It is another object of the present invention to provide a cross-connect switch which requires the minimum number of switch points to construct.
These and other objects of the present invention will become obvious to those skilled in the art from the following detailed description of the invention and the accompanying drawings.